51.pdf

IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE)

ISSN: 2278-1684, PP: 12-19 www.iosrjournals.org

Second International Conference on Emerging Trends in Engineering (SICETE) 12 | Page

Dr.J.J.Magdum College of Engineering, Jaysingpur

Governing Loads for Design of A tall RCC Chimney

M. G. SHAIKH, MIE1 , H.A.M.I. KHAN

2

1( Department of Applied Mechanics, Government college of Engineering Aurangabad (MS) 431001, India ) 2( Department of Applied Mechanics, Government college of Engineering Aurangabad (MS) 431001, India )

Abstract: Design of tall chimneys requires dynamic analysis for loads due to self weight, earthquake and

wind. Because of changes in the dimensions of chimney, structural analysis such as response to earthquake and

wind oscillations have become more critical. The present paper discusses analysis of reinforced concrete tall

chimney. The main focus is to compare the wind analysis result with that due to seismic one. Wind analysis is

done for along wind by peak factor method as well as by gust factor method and for across wind by simplified method as well as by random response method (shell completed case). The results obtained in above cases are

compared. The seismic analysis is performed using response spectrum method. Finally, the maximum values

obtained in wind analysis and seismic analysis are then compared for deciding the design values. Key words - Chimney, Seismic Analysis, Wind Loading, Wind Analysis

I. INTRODUCTION As large scale industrial developments are taking place all around, a large number of tall chimneys would be

required to be constructed every year. The primary function of chimney is to discharge pollutants into

atmosphere at such heights and velocities that the concentration of pollutants deemed harmful to the

environment are kept within acceptable limits at ground level. Due to increasing demand for air pollution, height

of chimney has been increasing since the last few decades, and these are valid reasons to believe that this trend

towards construction of taller chimneys will continue. However, chimneys being tall slender structures, they

have different associated structural problems and must therefore be treated separately from other forms of tower

structure.

Construction of such tall chimneys needs the better understanding of loads acting on them and of the

structural behavior, so that with the help of modern construction equipment and technique such as slip form,

reinforced concrete, the most favored material for chimney construction, could be used efficiently. The proper design and construction of such chimneys will create self standing structures to resist wind load and other forces

acting on them. It is a common practice to consider the effects of wind and earthquake separately in the design.

The present paper discusses analysis of reinforced concrete tall chimney. The main focus is to compare the wind

analysis result with that due to seismic one. Wind analysis is done for along wind and across wind (shell

completed case) and the results so obtained are compared with seismic analysis for deciding the design criteria.

II. HEADINGS 1. Description of Structure: A single flue reinforced concrete chimney is considered for the analysis situated in seismic zone III. The flue gas emission point will be 220 m above the finished floor level. The liner is

essentially constructed from structural steel and shall be hung from the liner support platform near the chimney

top. The liners are provided with resin bonded wool type thermal insulation; there will be several internal

platforms of structural steel provided along the height of the chimney. Except for the roof platform, all the other

internal platforms will have steel grillage of beams covered with galvanized gratings; internal platforms are

provided for enabling access to various elevations of the chimney and provide restraint to the steel liners.

External concrete platforms are supported by the chimney shell. The chimney roof shall however comprise of a

reinforced concrete slab supported over a grid of structural beams. The roof slab shall be protected by layer of

acid resistant tiles. The grade level slab shall be of reinforced concrete. An internal structural steel staircase

supported from the floor at the bottom with a guide support from shell is considered up to the support platform. There shall be rack and pinion elevator. Both the elevator and staircase will provide access to all internal and

external platforms.

The chimney height and top diameter are governed by exit velocity of gas and dispersion of effluent to

a larger area within specified limits of ground level concentration ref [1]. It is known by test that downwash can

be avoided if efflux velocity is greater than 1.5 times the wind speed for this reason, the chimney flue at top is

based on minimum exit velocity between 15 to 25m/sec, and the Indian code IS: 4998 gives an empirical




formula to calculate the chimney height. The external profile of the chimney shell is derived from the structural

consideration of the super structure and the foundation. The top portion to the extent possible is kept cylindrical

followed by linear slopes. The diameter of the chimney shell at the top is kept minimum possible allowing for

accommodation of the flue, staircase and the elevator. The bottom diameter of chimney is normally governed by

structural requirements, for single flue chimney an outside batter of 1 in 50 or 60 and a ratio of height to base

diameter in the range of 12 to 15 is provided. Single flue of structural steel is provided to discharge the flue gases and is hung from the liner support

platform near the chimney top. The shell rests on R.C.C. mat foundation of circular shape. 1.1 Details of the chimney are as follows

1. Height of chimney - 220 m

2. Outer dia at bottom - 18.36 m

3. Outer dia at top - 6.082 m

4. Thickness of shell at bottom - 0.5 m

5. Thickness of shell at top - 0.275 m

6. Grade of concrete - M30

7. Exit velocity of gas at top - 25.0 m/sec

8. Flue gas volume from the flue 340 cum/sec

9. Maximum flue gas temperature 135 degree centigrade. 10. Seismic Zone - III

11. Basic wind Speed - 44 m/sec (for Koradi)

12. Foundation Type - RCC circular mat

2. Description of Loading

2.1 Dead Load

Density of various materials considered for design

Concrete : 25 kN/m3

Insulation : 1 kN/m3

Soil : 18 kN/m3

Structural steel : 78.5 kN/m3 2.2 Live Load

5 kN/m2 will be considered for the design of internal and external platforms.

2.3 Wind Load

The following wind parameters are followed in accessing the wind loads on the structure.

Basic wind speed = 44 m/sec (for koradi as per IS: 875 (part3) 1987 ref [5])

Terrain category = 2

Class of structure = C

Risk coefficient k1 = 1.07

Topography factor k3 = 1.00

k2 factors shall be suitably taken along the height as given in Table – 2 and Table – 33 as per IS: 875 part-3

according to the method of analysis as per IS: 4998 (part1) 1992 ref [3].

Drag coefficient = 0.8 (for the unstraked region)

3. Earthquake Force Data Earthquake load for the chimney has been calculated as per IS: 1893 Part-1 2002 ref[4]. Accordingly the

relevant parameters are as follows:

Zone factor (Z) = 0.16

Seismic zone = III

Importance factor (I) = 1.75

Reduction factor (R) = 3

Therefore seismic coefficient (Ah) =

g

S

R

IZ a

2

Max horizontal seismic coefficient Ah= g

Sa3

75.1

2

16.0 = 0.047

g

Sa

Horizontal seismic coefficient Ah = 0.047 g

Sa




Accordingly a seismic coefficient value of 0.047 is considered and the g

SaValues corresponding to hard soil

are taken from Figure-2 of ref [4], Dynamic modulus of elasticity considered = 3.35 kN/m2 as per IS: 4998 part -

1 1992, Poisson ratio considered for concrete = 0.15

III. INDENTATIONS AND EQUATIONS

1. Analysis The analysis of R.C.C. chimney is carried out when it is subjected to wind forces and earthquake forces, at

various sections along its height as shown in the following figure.

The wind analysis is carried out as per methods given in IS 4998 (Part1) 1992 for along wind by Peak Factor

Method and Gust Factor Method, for across wind by Simplified Method and Random Response Method.

2. Analysis Procedure for Wind Load as per IS 4998 (Part 1) 1992 2.1 Peak Factor Method for Calculation of Wind Load

The along wind load or drag force per unit height of the chimney at any level is calculated from the equation

Fz = Pz CD Dz where Pz is design wind pressure obtained in accordance with IS 875 (Part 3): 1987, Z is height of

any section of the chimney in m measured from top of foundation, CD is drag coefficient of the chimney to be

taken as 0.8, and Dz is diameter of chimney at Z height.

The chimney is divided into twenty eight no of sections along its height and the lateral load due to wind at any

section is calculated by suitably averaging the loads above and below it. The moments are calculated from the sectional forces treating the chimney as a free standing structure.

2.2 Simplified Method for Response of Chimney

2.2.1 The amplitude of vortex excited oscillation, perpendicular to the direction of wind for ith mode of

oscillation is calculated by the formula:

(1): ηoi = sin

L

H

zzi

H

zziz

KS

C

d

dd

2

0

2

0

4

where ηoi = peak tip deflection due to vortex shedding in the ith mode of vibration in m, CL = peak oscillatory

lift coefficient to be taken as 0.16 H = height of chimney in m, Ksi = mass damping parameter for the ith mode of

vibration, Sn = Strouhal number to be taken as 0.2, Φzi = mode shape function normalized with respect to the

dynamic amplitude at top of the chimney in the ith mode of vibration.

2.2.2 The sectional shear force (Fzoi) and bending moment (Mzoi) at any height zo, for the ith mode of vibration,

are calculated from the following equations:

(2): Fzoi = 4 π2 f1

2 ηoi zzi

H

z

z dm 0

(3): Mzoi = 4 π2 f12 ηoi zzi

H

z

z dzzm 0

0

.

where f1 = natural frequency of the chimney in Hz in the ith mode of vibration, mz = mass per unit length of

the chimney at section z in kg/m.

Periodic response of the chimney in the ith mode of vibration is very strongly dependent on a dimensionless mass damping parameter ksi calculated by the formula:

(4): Ksi = 2.

2

d

m sei

where mei = equivalent mass per unit length in kg/m in the ith mode of vibration as defined, δs = logarithmic

decrement of structural damping = 2πβ, β = structural damping as a fraction of critical damping to be taken as 0.016, σ = mass density of air to be taken as 1.2 kg/m3, d = effective diameter taken as average diameter over the

top 1/3rd height of chimney in m.

2.2.3 The equivalent mass per unit length in ith mode of vibration (mei) is calculated by the formula:




(5): mei =

zzi

H

H

zziz

d

dm

.

.

2

0

0

2

2.3 Gust Factor Method for Calculation of Wind Load

The along wind response of a chimney is also calculated by the gust factor method. The use of the gust factor

method requires knowledge of hourly mean wind speed (HMW). Hourly mean wind speed at any height (z) is

obtained as per IS 875 (part3) 1987.

The along wind load as per unit height at any height z on a chimney is calculated from the equation, Fz = Fzm +

Fzf , where Fzm is the wind load in N/m height due to HMW at height z and is given by Fzm = Pz CD Dz , Fzf is

the wind load in N/m height due to the fluctuating component of wind at height z and is given by:

(6): Fzf =

H

zzm dzFH

z

H

G

0

2..

13

Pz = design pressure at height z, due to HMW is obtained as 0.6 V2 z (N/m2)

G is the gust factor which is calculated from the equation:

(7): G = 1 + gf r

SEB

where gf = peak factor defined as the ratio of the expected peak value to RMS value of the fluctuating load:

(8):

vT

vTg

e

e

flog2

577.0log2

(9):

2

1

1

1

3600

SE

B

fT

r = twice the turbulence intensity:

(10): r = 0.622 – 0.178 log10 H

B = background factor indicating the slowly varying component of wind load fluctuation:

(11): B = [1 + (H/265)0.63]-0.88 E = a measure of the available energy in the wind at the natural frequency of chimney:

(12): E =83.0

42.0

2

10

1

21.0

_

10

1

3301

123

HV

f

H

V

f

S = size reduction factor:

(13): S =

88.0

98.0

14.1

_

10

178.51

H

V

f

V10 = hourly mean wind speed in m/sec at 10m above ground level = Vb 2

_

k are as defined in IS 875

(part3):1987.

f1 = natural frequency of chimney in the first mode of vibration in Hz.

The Shear Force due to wind at any section is calculated by suitably averaging the loads above and below it. The

moments are calculated from the sectional forces treating the chimney as a free standing structure.

2.4 Random Response Method for Response of Chimney




Calculation of across wind load is made by first calculating the peak response amplitude at the first mode of

vibration.

The top one-third height of chimney is cylindrical that is with no taper. Therefore the modal response at a

critical wind speed Vcri = f1 d / Sn, is calculated by the formula.

(14): ηoi =

2

1

22

1

0

2

2

22

1

2225.1

ei

a

H

zzi

ein

iL

m

dkd

H

m

Ld

S

HdC

where ∩ = equivalent aspect ratio = H/d, CL = RMS lift coefficient to be taken as 0.12, L = Correlation length in

diameters which may be taken as 1.0 in the absence of field data, ka = aerodynamic damping coefficient to be

taken as 0.5.

The sectional shear force (Fzoi) and bending moment (Mzoi) at any height zo, for the ith mode of vibration, are

calculated from the following equations:

(15): Fzoi = 4 π2 f12 ηoi zzi

H

z

z dm 0

(16): Mzoi = 4 π2 f12 ηoi zzi

H

z

z dzzm 0

0

.

And seismic analysis of chimney is performed by Response spectrum in STAAD PRO 2007 software in which

the chimney is modeled as vertical cantilever structure fixed at the base having varying cross section area,

inertia mass along the height. The 220 m height of chimney is divided into 27 elements their masses lumped at

their centre of gravity having one degree of freedom. The discretization of chimney is shown in the following

figure Response spectrum analysis for the given acceleration response spectra for first five mode shape as shown in the following figure have been considered. 3. Results Graphs are the presentations of the results obtained by these methods for the 220 m height RCC chimney,

different graphs are prepared which show the variations as shown in fig 4 and 5 for different internal forces like

bending moment and shear force. IV. FIGURES AND TABLES







Fig 4: Variation of Bending Moment along the Height of Chimney

Fig5: Variation of Shear Force along the Height of Chimney

Table. Floor Weight Calculation at Different Platform level

V. CONCLUSION The effect of wind forces is quite significant as compare to earthquake forces over 220 m height R.C.C chimney.

The geometry of chimney has to be so chosen that the deflection of chimney at the top is within permissible

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728

No of Section along the Height

Ben

ding

Mom

ent i

n kN

-m (T

hous

and)

Peak Factor Values (along)

Gust Factor Values (along)

Simplified Values (across)

Random Response Values

(across)

Seismic Values

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728

No of Section along the Height

Shea

r Fo

rce

in k

N (H

undr

ed)

Peak Factor Values (along)

Gust Factor Values (along)

Simplified Values (across)

Random Response Values

(across)

Seismic Values

Floor No

(1)

Elevation

(m) (2)

Diameter

(m) (3)

Shell Thickne

ss (m) (4)

Area (m2)

(5)

Deduction for flue

(m2) (6)

Net Floor

Area (m2)

(7)=(5)-(6)

Unit wt for floor

load calculatio

n kN/m2

(8)

Floor wt kN

(9)=(8)x(7)

Liner wt

kN (10)

External

Platform kN (11)

Total kN

(12)

7 215 6.082 0.275 24 12.5 11.5 9.0 103.5 - - 104.0

6 210 6.082 0.275 24 12.5 11.5 4.0 46.0 2400.

0 -

2450.0

5 170 6.082 0.275 24 12.5 11.5 2.0 23.0 - 100.0 123.0

4 120 9.59 0.339 62.5 12.5 50 2.0 100.0 - 150.0 250.0

3 72 13.54 0.412 127.

5 12.5 1.5 2.0 230.0 - 200.0 430.0

2 30 17.483 0.484 214.

4 12.5 201.9 2.0 410.0 - 260.0 670.0

1 20 17.921 0.492 225.

5 12.5 213 2.0 430.0 - - 430.0




limits. The presence of gust wind over a considerable height of chimney plays important role as the forces

obtained by gust factor method are quite high along the sections considered except the top four sections where

the forces obtained in seismic analysis are higher. Elsewhere, the effect of earthquake forces seems

comparatively lesser along the height of the chimney. The cross wind analysis is taken care of by considering

first mode of oscillation as the critical wind speed is well within the design wind speed for the first two modes.

Having known this, a given tall reinforced concrete chimney can now be designed for respective wind and seismic forces obviating the need for empirical formulae.

REFERENCES [1] S. N. Manohar,, tall chimneys design and construction, 1985, TATA McGraw – Hill Publishing Company Limited

[2] G. M. Pinfold, reinforced concrete chimneys and towers, A view point publication, UK, 1984

[3] I.S 4998 (Part1) – 1992, Indian Standard Code of Practice for Criteria for Design of Reinforced Concrete Chimneys. Part 1:

Assessment of Loads. (Bureau of Indian Standard, New Delhi)

[4] IS 1893(Part1) – 2002, Indian Standard Code of Practice for Criteria for Earthquake Resistant Design of Structures. Bureau of

Indian Standard, (New Delhi)

[5] IS 875 (Part3) – 1987, Indian Standard Code of Practice for Criteria for Design Loads (Other than Earthquake) For Buildings

and Structures. (Bureau of Indian Standard, New Delhi)

[6] K. Suresh Kumar and G. N. V. Rao, Wind Loading over the Top Portions of Tall Stacks with and Without External Landing

Platforms, Journal of Wind Engineering and Industrial Aerodynamics, Vol. 51, Page 319-338, July 1993

[7] Lawrence C Maugh and Wadi S. Rumman, Dynamic Design of Reinforced Concrete Chimneys, ACI Journal, Vol. 64, No. 47,

Page 560-567, September 1967

[8] S. R. Joshi, N. S. Pendse, V. T. Patilkakad, Some Special Aspects of the Design and Analysis of Tall Chimneys, Irrigation and

Power Journal, Vol. 42, No. 1, January 1985

[9] K. S. Babu Narayan, Subhash C. Yarogal, and Yukio Tamura, “Interaction Envelops For Limit State Design of Chimneys”,

Fourth International Symposium on Computational Wind Engineering, Yokohama, 2006

[10] J. L. Wilson, Code Recommendation for the aseismic Design of Tall Reinforced Concrete Chimneys, CICIND’s Report

Australia, Bibao, Vol. 16, No. 2, September 2000

[11] N. S. Pour, Indrajit Chowdhary, Dynamic Soil – Structure Interaction Analysis of Tall Multi flue Chimneys Under Aerodynamic

and Seismic Force, Twelfth International Conference of IACMAG India, Goa, Page 1-6, October 2008

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